This article, whose materials are extracted from a wider project on the doctrinal and humanitarian significance of the 1899/1907 Martens Clause, reviews the strengths and limitations of competing interpretations and judicial applications of this Clause. It identifies four distinct, if interrelated, approaches to defining its meaning and scope assessing each in turn. We take issue with recent scholarship that restricts its applicability in various ways that deny its status as a separate and distinct legal principle of direct and independent applicability to organized atrocities against civilians. We also dispute the view that this Clause is best interpreted as an aide to judicial interpretation, rather than as an independent source of international criminal law, by showing that this interpretation is inconsistent with a number of important cases whose authority appears to be well established and unobjectionable. Furthermore, the moral imperatives that clearly shape the language of the Clause and have been realized in many of its accumulated judicial applications, positively require this measure to be interpreted and applied as a freestanding legal norm—albeit one that has to operate as supplement for, rather than alternative to, other more specific legal rules and principles.